Hornets usually eat live insects.
Some other large wasps are sometimes referred to as hornets such
as the Bald-faced hornet which visits flowers, especially in late
summer, and can be minor pollinators. In the fall, yellowjackets
may be attracted to human foods and food wastes.

Many bees and wasps do pollinate, some to a greater degree than
others See this site:
http://pollinator.com/identify/whatsbuzzin.htm. I found great info
on this website; almost all are beneficial in some way. Many wasps
predate on garden pests, such as the tomato hornworm.